The present invention relates to a double window display apparatus which is capable of displaying two images at right and left portions of one image display screen and varying display magnification of each of right and left images.
This application is based on Japanese Patent Application No. 08-97033, filed Apr. 18, 1997, the content of which is incorporated herein by reference.
An image displaying apparatus capable of displaying two images on one screen has been known. Although only picture-in-picture method has been known with which a sub-screen is inserted into a main screen, a trend of widening (in the lateral direction) the television image has, in recent years, permitted a double window system to be adapted to the wide screen such that two image are displayed in parallel in the lateral direction of the screen. That is, a main screen is displayed in the left-hand portion of the screen, while a sub-screen is displayed in the right-hand portion of the same.
On the other hand, a color television signal of the NTSC (National Television System Committee) method which has been employed in Japan has an aspect ratio of the screen of 4:3. The HDTV (High Definition Television), which is in a research and development stage, recommends a more laterally-wide aspect ratio of 16:9. As a method enabling a program in the form of the wide screen to be received by the conventional NTSC television tuner, a letter box system has been employed. The letter box system is a system structured as shown in FIG. 1 such that a laterally-elongated window having a wide aspect ratio of 16:9 is displayed in the central portion of a screen having an aspect ratio of 4:3 while making the upper and lower peripheral portions to be non-image regions.
FIG. 2 shows the structure of a conventional television tuner employing the letter box system to display two images on a screen comprising, for example, a CRT monitor. First composite video signal S1 for the main screen supplied to a first input terminal 11 is supplied to a Y/C (luminance/chrominance) separation circuit 12 and a frame synchronizing/sub-screen processing circuit 13. The Y/C separation circuit 12 separates the first composite video signal S1 into luminance signal Y1 and chrominance signal C1. The luminance signal Y1 and the chrominance signal C1 are supplied to a horizontal compression circuit 14. The horizontal compression circuit 14 compresses, to 1/2, the input luminance signal Y1 and the chrominance signal C1 in a horizontal direction so as to output compressed luminance signal Y2 and chrominance signal C2. The output luminance signal Y2 is supplied to one of input terminals of a selector 15, while the chrominance signal C2 is supplied to one of input terminals of a selector 16.
The second composite video signal S2 for the sub-screen supplied to the second input terminal is, by a Y/C separation circuit 18, separated into luminance signal Y3 and chrominance signal C3 so as to be supplied to a frame synchronizing/sub-screen processing circuit 13. The frame synchronizing/sub-screen processing circuit 13 initially synchronizes the first composite video signal S1 and the luminance signal Y3 and the chrominance signal C3, which are included in the second composite video signal, so as to correct deviation in the synchronization between the two images, if necessary. Then, the frame synchronizing/sub-screen processing circuit 13 subjects the luminance signal Y3 and the chrominance signal C3 output from the Y/C separation circuit 18 to a sub-screen process to form a right-half image of the double screen. Luminance signal Y4 subjected to the sub-screen process is supplied to another input terminal of the selector 15, while chrominance signal C4 subjected to the sub-screen process is supplied to another input terminal of the selector 16.
The frame synchronizing/sub-screen processing circuit 13 also outputs selection signal S3. The selection signal S3 is supplied to the selectors 15 and 16. The selection signal S3 has a low level during a second half of the horizontal scanning period.
In response to the selection signal S3 supplied from the frame synchronizing/sub-screen processing circuit 13, the selector 15 alternately and selectively outputs the luminance signal Y2, which is included in the first composite video signal, and the luminance signal Y4, which is included in the second composite video signal in each (1/2) H period ((1/2) horizontal scanning period). When the level of the selection signal S3 is high (in the first half portion of the horizontal scanning period), the selector 15 selects the luminance signal Y2 which is included in the first composite video signal. When the level is low (in the second period of the horizontal scanning period), the selector 15 selects the luminance signal Y4 which is included in the second composite video signal. Selected luminance signal Y5 is supplied to a first input terminal of a matrix circuit 19.
In response to the selection signal S3 supplied from the frame synchronizing/sub-screen processing circuit 13, the selector 16 alternately and selectively outputs the chrominance signal C2 which is included in the first composite video signal and the chrominance signal C4 which is included in the second composite video signal in each (1/2) H period ((1/2) horizontal scanning period). The selector 16 selects the chrominance signal C2 which is included in the first composite video signal in the first half portion of the horizontal scanning period in which the level of the selection signal S3 is high and selects the chrominance signal C4 which is included in the second composite video signal in the second half portion of the horizontal scanning period in which the foregoing level is low. Selected chrominance signal C5 is subjected to a color demodulation process by an I/Q separation circuit 23 so as to be separated into color difference signals I and Q which are then supplied to second and third input terminals of the matrix circuit 19.
The matrix circuit 19 subjects the signals Y5, I and Q to a matrix conversion process so as to generate red, green and blue color signals R, G and B to output the same to a monitor 22 which is, for example, a CRT (Cathode Ray Tube) display device.
The image to be displayed on the monitor 22 may be retained to be an image of the letter box system as shown in FIG. 1. As an alternative to this, the image is expanded in the vertical direction to omit the upper and lower non-image portions, as shown in FIG. 3. When the image is expanded vertically, the microcomputer 20 calculates the expansion ratio in the vertical direction required to widen the image to the overall size of the screen. Then, the microcomputer 20 outputs mode switch signal MS including the vertical expansion ratio to a deflection-angle control circuit 21. In accordance with the mode switch signal MS, the deflection-angle control circuit 21 generates horizontal deflection signal SH and vertical deflection signal SV. The deflection-angle control circuit 21 controls the vertical deflection signal SV in accordance with the vertical expansion ratio to enlarge the deflection angle. The horizontal deflection signal SH and the vertical deflection signal SV are supplied to the monitor 22 so as to be subjected to a process for deflecting electron beams. If the microcomputer 20 outputs, to the deflection-angle control circuit 21, mode switch signal MS including vertical compression ratio in stead of the vertical expansion ratio, the deflection-angle control circuit 21 controls the vertical deflection signal SV to reduce the deflection angle in accordance with the vertical compression ratio.
FIG. 4 shows states of the first composite video signal and the second composite video signal. That is, the luminance signal Y2 and the chrominance signal C2 which are included in the first composite video signal and output from the horizontal compression circuit 14 form a left-half image of the double screen, as shown in the uppermost portion shown in FIG. 4. The signal components exist in only the first half portion of one horizontal scanning period. The luminance signal Y4 and the chrominance signal C4 which are included in the second composite video signal and output from the frame synchronizing/sub-screen processing circuit 13 form a right-half image of the double screen, as shown in the central portion shown in FIG. 4. The signal components exist in only the second half portion of one horizontal scanning period. The selector 15 alternately and selectively outputs the luminance signal Y2 which is included in the first composite video signal and the luminance signal Y4 which is included in the second composite video signal in each (1/2) H period in response to the selection signal S3. As shown in the lowermost portion shown in FIG. 4, the selected luminance signal Y5 has two types of luminance signal components in one horizontal scanning period. Also the selector 16 performs a similar operation so that chrominance signal C5 output from the selector 16 has two types of color components in one horizontal scanning period.
As described above, a display device of a type comprising the CRT is able to expand/compress an image in the vertical direction by controlling the deflection angle of the electron beams. However, control of the deflection angle causes the expansion and compression ratios not to be changed in each of the right and left windows of the double window. In an example case where the main screen and the sub-screen display different images (NTSC system and the HDTV system), it is not possible vertically expand only the HDTV image. Moreover, one of the images cannot be expanded and compressed by using the other image as a reference.
On the other hand, a display device comprising liquid crystal device is, differentially from the CRT, able to expand or compress the image by changing the number of the scanning lines in place of the control of the deflection angle. The letter box system uses 360 lines/frame in the central portion of the screen among an effective number of scanning lines of about 480 lines/frame for the NTSC system to transfer an image having the aspect ratio of 16:9. When an image of the letter box system is expanded and displayed on a television tuner having a wide screen (having an aspect ratio of 16:9), video signals for 360 effective scanning lines must be converted into video signals for 480 effective scanning lines, that is, a so-called 3-4 conversion must be performed.
In a case where a display device of the foregoing type comprising the LCD displays, on a double window, an image of the NTSC system having the aspect ratio of 4:3 and a wide image of the HDTV system having the aspect ratio of 16:9, two images can be processed with different expansion ratios or compression ratios. However, two image display devices must be provided for the right window and the left window, respectively. Thus, a field memory having a great capacity must be provided, thus causing a problem to arise in that the size of the structure and the cost cannot be reduced.
As described above, there arises a problem in that the double window system for displaying two images by dividing the display screen into the right and left windows cannot easily expand or compress the right and left images in the vertical direction with individual expansion ratios or compression ratios.